Saw a thing on slashdot a few weeks ago which would be perfect for this... someone essentially enlarged a printer and put a spray-paint can on it instead of an ink cartriage.
The spray cartriage could travel in a 2D plane - something crazy like 20x20 feet. And the thing could be mounted vertically and spray a wall ;-)
[Edit]Found the link:
Giant "Inkjet Printer" [/Edit]